1. Field of the Invention
The present invention relates to an inverted-F metal plate antenna that can be suitably used, for example, as a small and inexpensive internal antenna for communication.
2. Description of the Related Art
Inverted-F metal plate antennas formed by bending metal plates are often used, for example, as internal antennas for communication, since inverted-F metal plate antennas can be manufactured relatively inexpensively and are advantageous for reducing size and height, and exhibit favorable antenna characteristics.
FIG. 5 is a perspective view of a common inverted-F metal plate antenna that has been known. Referring to FIG. 5, an inverted-F metal plate antenna 1 is fixed on a ground conductor surface 2 composed of a conductor plate or conductor foil. The inverted-F metal plate antenna 1 is formed by bending a sheet of metal plate. The inverted-F metal plate antenna I is composed of a radiating conductor plate 3 disposed opposing and in parallel with the ground conductor surface 2, a power-feeding conductor plate 4 extending substantially perpendicularly from an outer edge of the radiating conductor 3 and connected to a power-feeding circuit that is not shown, and a shorted conductor plate 5 extending substantially perpendicularly from an outer edge of the radiating conductor 3 and connected to the ground conductor surface 2. In the conventional inverted-F metal plate antenna 1, the lengthwise dimension of the radiating conductor 3 is chosen to be approximately one fourth of the resonant length so that when a predetermined high-frequency electric power is supplied to the radiating conductor plate 3 via the power-feeding conductor plate 4, the radiating conductor plate 3 is excited, allowing transmission and reception of signal waves in a predetermined frequency band associated with the resonant length.
The inverted-F metal plate antenna 1 constructed as described above, however, has a narrow resonant frequency band (bandwidth) in which the voltage to stationary wave ratio (VSWR) is not larger than 2 and the amount of reflection is not larger than −10 dB. For example, since the frequency band used in a wireless LAN that operates in the 5-GHz band is rather wide, an antenna for the wireless LAN must have a bandwidth at least as wide as 300 MHz, and preferably 500 MHz or larger. The inverted-F metal plate antenna 1 is not suitable for practical use since its bandwidth is only as wide as approximately 200 MHz.
In order to overcome the problem, a type of inverted-F metal plate antenna has been proposed in which another metal plate (shorted conductor plate) is connected and fixed at a position that is deviated by a predetermined amount from the center of the radiating conductor plate and in which the metal plate is connected and fixed on a ground conductor surface. This type of inverted-F antenna is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 11-041026, at page 3 and in FIG. 1. In such an arrangement in which a shorted conductor plate is connected and fixed at a position that asymmetrically divides a radiating conductor plate in two, distances of the shorted conductor plate to substantially parallel two sides of the radiating conductor plate differ. Thus, two different resonant modes at different frequencies, reflecting the difference in distance, can be generated when power is supplied. This serves to increase the bandwidth of an inverted-F metal plate antenna.
The related art disclosed in Japanese Unexamined Patent Application Publication No. 11-041026 is effective for increasing the bandwidth of an inverted-F metal plate antenna. However, since a separate shorted conductor plate must be connected and fixed at a predetermined position of a radiating conductor plate by soldering or the like, manufacturing cost increases compared with a common inverted-F antenna, such as the one shown in FIG. 5, that can be formed by bending a sheet of metal plate. Furthermore, according to the related art disclosed in Japanese Unexamined Patent Application Publication No. 11-041026, since a shorted conductor plate is disposed at a position where a radiating conductor is divided in two, the lengthwise dimension of the radiating conductor plate must be chosen to be approximately one half of the resonant length. This prohibits miniaturization of the inverted-F metal plate antenna.